Our data indicate that the cannabinoids produce antinociception by interaction with kappa opioids in the spinal cord. Cannabinoid-induced antinociception is blocked by the kappa antagonist nor-binaltorphimine and dynorphin antisera. Cannabinoids exhibit bi-directional cross-tolerance to kappa agonists and dynorphins are cross-tolerant to THC. The antinociceptive effects of THC are attenuated by antisense to the kappa-1 receptor. Prevention of metabolism of dynorphin to leucine enkephalin prevents the enhancement of opioid antinociception by the cannabinoids. Thus, leucine enkephalin production may result in the cannabinoid-induced enhancement of opioid antinociception. However, the mechanism of action of anandamide, its activity in modulating pain, and its interactions with opioids remains unclear and appears to differ somewhat from that of the cannabinoids. The proposed studies will provide insight into the distinct mechanisms by which the body produces and utilizes endogenous substances in the modulation of nociception. The "Specific Aims" in this proposal are based upon a preponderance of in vivo data indicating an interaction of acutely administered cannabinoids with the kappa opioids, and are designed to compare and contrast the acute versus the chronic effects of cannabinoids on three interrelated parameters: 1) alterations in the release of dynorphin; 2) alterations in kappa receptor number and/or affinity and 3) alterations in the number or affinity of cannabinoid receptors at discrete spinal sites. Given the differences in profile of the mechanism of action of THC, the synthetic cannabinoid CP55,940, and anandamide, all three drugs will be compared and contrasted in our test systems. The rationale for our proposal is an outgrowth of our exciting initial finding that the cannabinoids differ in that they generally fall into two categories -- those that enhance the antinociceptive effects of morphine in the spinal cord (delta9-THC, for example) and those that enhance the effects of morphine in the brain (CP55,940 for example). Our hypotheses will be evaluated by the quantitation of dynorphin release from spinal cord, followed by measurement of mRNA levels of prodynorphin, kappa1- and delta-opioid receptors, and CB1 and CB2 receptors. Additionally, radioligand binding for spinal kappa receptors, autoradiographic analysis of CB1 receptors in brain and spinal cord regions, Western immunoblotting for CB1 and CB2 receptors and immunocytochemistry of cannabinoid receptors in spinal cord and brain will be performed.